Course: Oilseed rape | Last Updates: 9th October 2015
Hybrid varieties of agricultural and horticultural crops are increasingly being grown as plant breeders find desirable genetic traits they can exploit to bring agronomic advantages.
Oilseed rape was one of the first arable crops to benefit, though the hybrids of 20 years ago were not specifically bred for UK conditions and thus had limited success here. Varieties from continental Europe tended to establish slowly and were late to take off in the spring, then became excessively tall, proving difficult to manage in the field.
Plant breeders responded by focusing on characteristics suited to our maritime climate, and have since managed to incorporate numerous genetic attributes. The process is quite different to conventional breeding of open-pollinated types, which traditionally takes three to four years longer, as characteristics are selected and purified through back-crossing.
Hybrids are produced at the seed production stage by crossing two parental lines that are ideally genetically distinct, to produce offspring with hybrid vigour or heterosis. The cross combines the chromosomes of the respective parent lines to produce a uniform first generation where all the plants are genetically the same. The seed is usually referred to as F1 seed.
The process involves several genetic techniques to produce a female parent or "male-sterile" to prevent self-fertilisation, and restorer genetics to confer fertility back to the F1 plant so that it can itself produce pollen and thereby a crop.
As an example, take the semi-dwarf hybrid, typically a cross between a very tall "female line" and a very short male line with homozygous dominant Bzh dwarfing genes. The cross produces F1 seed that will grow to produce a uniform crop of intermediate height, lodging-proof with the high yields or other characteristics from the female parent.
Note that if the seed from such crops is kept and resown, the next crop would be of mixed plant height and lack uniformity. It would grow, but the benefits of the original hybrid-cross would be lost. This population segregation is why growers should not save hybrid seed. It is against the law to establish a commercial crop using farm-saved seed from a hybrid variety.
Plant breeders aim to produce elite parental lines from diverse gene pools and test parental combinations and, where possible, combine desirable traits. A good parental line can be used in many varieties.
While there is little to separate the yield performance of hybrids and conventional varieties on the HGCA Recommended List, there is evidence to suggest a difference in performance at lower commercial seed rates in farm-scale trials and on farms. Furthermore, hybrids offer improved establishment and exclusive traits.
Early hybrids, such as Excalibur, had a distinct "growiness" that suited the demands of changing establishment systems and canopy management through the adoption of low seed rates. While one cannot definitively associate vigour with hybrids, evidence suggests the most vigorous hybrids are more aggressive than the most vigorous conventional varieties in both the autumn and the spring.
A trait is what gives a variety its distinguishing quality, differentiating it from others. Traits differ from the general varietal characteristics described on the HGCA Recommended List with a rating. Two examples present in a succession of hybrids are very high levels of genetic stem canker resistance, as in Fencer, and pod shatter resistance.
Traits are not specific to hybrids – for instance, the market has seen the recent introduction of turnip yellows virus resistance in one conventional variety. However, plant breeders gain better intellectual property and royalty protection longer term, which is allowing breeders to invest exclusively in hybrid development.
Genetic improvements in both conventional and hybrid varieties have seen seed oil contents rise in a decade from about 43% to 46%. Over the past eight years, the quality oils of conventional Hear (high-erucic acid rape) and Holl (high-oleic, low-linolenic) varieties such as Hearty and V140OL have been replicated in hybrids such as Palmedor and V316OL, with a significant gain in gross output and agronomic improvement.
Another benefit introduced through hybrid breeding is herbicide tolerance, expressed in Clearfield varieties, which may have further implications as herbicide options are reduced.
Choosing the right variety for the market, the situation and management strategy is more important than choosing between hybrids and conventionals. Agronomic profile is arguably more important than potential yield.
Also consider sowing date and challenges the crop is likely to face, such as flea beetle attack, disease risk and the need to spread workloads at harvest.
Hybrids were initially advantageous on lower-potential soils, but are now suited to all soil types and situations. They are not just about fast development to improve establishment; slow development can be important for fertile land or where drilling particularly early in August. The standing ability and stem stiffness offered by new semi-dwarfs such as DK Sensei can be useful in these situations.
Disease resistance in hybrids is genetically stable. For example, the RLM7 phoma stem canker resistance gene, first available in the hybrid Excel, has proved to be sustainable. With this as a base line trait, improved light leaf spot resistance was introduced and is expressed in newer hybrids currently in development by breeders such as Bayer Cropscience.
Selecting for high combined disease resistance reduces dependency on fungicides. Improved overall disease resistance is evident in new hybrids such as Alizze and Harnas, and conventional varieties such as Elgar.
Markets are increasingly driving oilseed rape variety decisions. For example, this year there has been a three-fold increase in the contract acreage for Holl varieties, driven by increased market demand and the desire to secure a premium to offset lower commodity prices. There are now several Holl hybrids with differing agronomic profiles.
Where cruciferous weeds closely related to oilseed rape have become a burden, consider a hybrid with Clearfield technology, denoted with the suffix CL after the variety name. Clearfield varieties can tolerate imidazolinone (IMI) herbicides such as imazamox, commonly used with metazachlor.
Growing hybrids is a relatively simple process, and with the exception of higher seed costs, they are generally no more expensive to grow than conventional varieties.
Seed rates are typically 20% lower than for conventional varieties, which compensates for higher seed production costs. The average seed rate for hybrids is 50 seeds/sq m, though many growers use 40 seeds/sq m if soil and conditions are favourable or 60 seeds/sq m if the seed-bed is cloddy or it is an exposed site.
The extra vigour of fast-developing hybrids can help to reduce the effect of poor conditions at establishment, pigeon damage or flea beetle attack. However, take steps to control volunteers. Hybrids can be affected more by oilseed rape volunteers than conventional varieties. This is a particular risk for Holl crops as it can result in the loss of the specialist oil premium.
Hybrids work equally well in both strip-till or more conventional establishment systems and benefit from placed seed-bed fertiliser. The principles of canopy management should be followed with a target of 3-3.5 GAI, based on 50kg N/ha for every unit of GAI plus additional nitrogen depending on yield potential.
Vigorous hybrids resist the development of phoma stem canker, both physically and genetically. Light leaf spot is arguably a more progressive and damaging disease, though less straightforward to control. Hybrid or conventional, the best solution is to use good genetic resistance alongside a protectant fungicide. Crops continue to need protecting against sclerotinia as there is no genetic solution at present.
The combining benefits attributed to the semi-dwarf hybrids, seen as a major breakthrough a decade ago, have not been realised because hybrids are generally not much taller than conventional types and are easier to manage through appropriate use of growth regulators.
The pod shatter resistance found in some hybrid varieties has a sound genetic basis. Trials have shown reduced seed shedding, lower volunteer populations and reduced slug grazing in the succeeding crop. In resistant varieties, there is no justification for a pod sealant application pre-harvest.
There is no need to store or market hybrids separately from conventional, with the exception of Hear and Holl crops which must be segregated, stored and moved separately.
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